In the US the 'S' code was used for both years.

In Canada the VIN's started with the year of the truck.

Take a look at this to decode your VIN.

http://www.chevytrucks.org/tech/canadavin.jpg

I do not know of any diffences between 58 and 59's.

What are your plans for the truck? Any pictures available?

Blaine Dumkee
59 GMC 9314
Fort Smith NT
www.flickr.com/photos/northerngmc/